> yum list installed | grep php

php.i386                             5.3.6-3.el5.art                   installed
php-cli.i386                         5.3.6-3.el5.art                   installed
php-common.i386                      5.3.6-3.el5.art                   installed
php-devel.i386                       5.3.6-3.el5.art                   installed
php-gd.i386                          5.3.6-3.el5.art                   installed
php-mbstring.i386                    5.3.6-3.el5.art                   installed
php-mysql.i386                       5.3.6-3.el5.art                   installed
php-pdo.i386                         5.3.6-3.el5.art                   installed
php-pear.noarch                      1:1.7.2-2.el5.art                 installed

> php -v

PHP 5.3.6 (cli) (built: Apr 19 2011 12:28:40)
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies

> yum list | grep zend

php-zend-guard-loader.i386                 5.5.0-3.el5.art             atomic
php-zend-optimizer.i386                    1:3.3.9-2.el5.art           atomic


> yum install php-zend-optimizer.i386

Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * atomic: www7.atomicorp.com
 * base: mirror.sov.uk.goscomb.net
 * extras: mirror.sov.uk.goscomb.net
 * rpmforge: apt.sw.be
 * updates: mirror.sov.uk.goscomb.net
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package php-zend-optimizer.i386 1:3.3.9-2.el5.art set to be updated
--> Processing Conflict: php conflicts php-zend-optimizer
--> Finished Dependency Resolution
php-5.3.6-3.el5.art.i386 from installed has depsolving problems
  --> php conflicts with php-zend-optimizer
Error: php conflicts with php-zend-optimizer
 You could try using --skip-broken to work around the problem
 You could try running: package-cleanup --problems
                        package-cleanup --dupes
                        rpm -Va --nofiles --nodigest